The present invention relates generally to implantable medical devices, and more particularly to improved implantable cardioverter/defibrillator devices for providing greater efficiency, safety and energy conservation by delivering a defibrillating shock waveform to the implant patient's fibrillating heart at a time of substantially lowest defibrillation threshold (DFT).
Implantable defibrillators are intended to detect events of fibrillation quickly and accurately, and to respond rapidly with an effective defibrillation therapy. Typically, these types of devices are designed to detect pathologic tachycardia and fibrillation events, and, in response, to initiate delivery of an electrical shock waveform (sometimes referred to herein simply as "shock") of appropriate energy and shape to cardiovert or defibrillate the patient's heart accordingly.
Clearly, it is important in the case of any implantable, battery-operated medical device to provide design and operation for energy conservation within the constraints of its therapeutic function to enable sufficient energy reserve for a reasonable lifetime of the device, and to allow for a reduction in size and weight to the lowest possible level. This is particularly important for automatic defibrillators, which may be required to expend comparatively large amounts of energy each time an episode of actual or imminent fibrillation is detected. The aim is that the designated therapy be delivered at a point in time adequate for successful termination of the dysrhythmia, to return the heart to normal sinus rhythm with the least expenditure of energy. Generally speaking, the optimum time for delivering a defibrillating shock is when the DFT is likely to be lowest within a suitable window of earliest opportunity. There is no value to a quest to save energy which imposes a waiting time that could cost the patient's life.
U.S. Pat. No. 4,384,585 to Zipes describes synchronous cardioversion in which the possibility exists to induce fibrillation by non-synchronous shocking. The synchronous cardioversion is intended to deliver the shock at a time when the bulk of the cardiac tissue is depolarized and in a refractory state. Non-synchronous cardioversion is sought to be avoided to preclude delivery of cardioverting energy during the vulnerable T-wave portion of the cardiac cycle. The patient's electrogram is used to detect depolarizations of the cardiac tissue and to produce a corresponding sense signal. Additional detection criteria are applied to determine whether a tachyrhythmia is present. The detection circuitry determines the time interval between successive cardiac depolarizations, and initiates a discharge of energy stored in an output storage capacitor if either the average detected heart rate is above a preset threshold for a specified period of time or the rate accelerates by a preset amount. Alternatively, the device detects a departure of selected beats from a historic data base of successive R-R intervals stored in the device memory; or performs a waveform analysis of the electrogram information with pattern recognition of time domain or frequency domain characteristics of the tachyrhythmia signal. The shock is delivered synchronously with the occurrence of an R-wave after specified criteria are met.
U.S. Pat. No. 4,996,984 to Sweeney describes a defibrillation technique in which fibrillation cycle length (average time interval between successive depolarizations) is measured and therapy is delivered in the form of multiple bursts of electrical current timed according to that cycle length. When cardiac tissue cells are activated, the normal electrical gradient constituting the voltage difference within and outside the cell collapses, or depolarizes. The depolarization propagates from cell to cell, the collapse of each initiating the collapse of the next in a moving wavefront. As a cell is depolarized it immediately begins repolarization to reestablish a voltage difference sufficient once again for depolarization, during the course of which the tissue is refractory. During the refractory period, the cell is incapable of responding to a stimulus. According to the multiple burst defibrillation technique of the '984 patent, the successive bursts are timed to occur at successive depolarizations at a designated site in the myocardium, so that the time interval between bursts is adjusted to correspond to the fibrillation cycle length. The cycle length is determined using cross-correlation, auto-correlation, fast Fourier transformation, counting R-waves of the electrocardiogram over a fixed time period, and determining the R-R intervals of individual cardiograms.
An improved technique for timing the delivery of defibrillating shocks is disclosed in co-pending patent application Ser. No. 310,281, filed Sep. 14, 1994 ("the '281 application") of the same applicants as in this case, commonly assigned herewith. The implanted device delivers the cardioverting or defibrillating therapy to the implant patient's heart timed for rapid and successful termination of a dysrhythmia in an energy efficient manner. To that end, changes are observed in characteristics of the patient's cardiac activity during fibrillation, such as amplitude and frequency of the ECG signal detected by intracardiac, transthoracic, or surface type techniques. The discrete ECG signal detected during fibrillation from intracardiac electrodes, for example, indicates the status of excitability or nonexcitability of the major parts of the heart. As the cardiac activity undergoes several fibrillatory cycles, a major fibrillation wavefront was found to drive the muscular masses through a regular cellular cycle of depolarization and repolarization, with the result that different masses of the heart have different status of absolute refractoriness, relative refractoriness and full excitability. It was determined that this regular cycle of the cells in a particular mass of tissue of the fibrillating heart (or with activity closely related thereto, such as an accelerating pathologic tachycardia or flutter) can serve to identify a point in the fibrillation cycle at which the heart has the lowest DFT and is therefore most susceptible to defibrillation by application of a relatively low energy shock.
In essence, the amplitude and frequency of the intrinsic ECG signal of a fibrillating heart undergo change according to a regular cycle which is detectable from the ECG wave envelope (e.g., by an intracardiac technique), as the major fibrillation wavefront propagates, so that the defibrillating shock may be timed for synchronous application at or near the point in time of highest amplitude of the ECG signal and lowest frequency of discrete intracardiac potentials, consistent with lowest DFT.
In an embodiment disclosed in the '281 application, detection criteria employed by the implanted device are used to sense imminent or ongoing fibrillation. At that time, the output storage capacitor(s) of the device commence charging, and while the charging is underway the phasic variations in amplitude and frequency of the patient's ECG are detected by pattern recognition, in preparation for timing the delivery of a shock waveform from the capacitor(s) at the optimum point in the regular cellular cycle of the cardiac activity. After a sufficient number of successive intervals of high amplitude, low frequency cardiac activity have been observed to indicate a trend, the capacitor(s) arc discharged to deliver a shock at the very next interval of increasing amplitude of the ECG signal, i.e., in substantial synchronism with a point of increasing amplitude of the intrinsic ECG signal and decreasing frequency of discrete depolarizations. The scanning of cardiac events is conducted over a search period of sufficient length to identify an optimum point in the cycle for delivery of the shock. The optimum point may occur where either or both of the amplitude of the intrinsic ECG signal and the interval between occurrences (i.e., reciprocal of frequency) of intracardiac discrete ECG depolarizations are increasing. Both parameters may be increasing, or only one, but the desire is to trigger the shock at a point where the ECG signal amplitude is likely to be greatest. The device may be implemented to calculate the quotient of amplitude over frequency to ascertain optimum timing.
It is the principal aim of the present invention to provide further improvements in methods and devices for timing the delivery of defibrillating shocks to achieve successful defibrillation rapidly and with relatively low energy.